1. Field of the Invention
The present invention relates to a cathode ray tube. More specifically, it relates to a cathode ray tube having a shadow mask stretched with the application of tension in one direction.
2. Description of the Prior Art
In a color cathode ray tube, a phosphor screen formed on an inner surface of a face panel is irradiated with electron beams emitted from an electron gun, and a desired image is displayed. At the side of the electron gun of the phosphor screen, a shadow mask acting as a color selection electrode is provided at a predetermined space from the phosphor screen. The shadow mask has a large number of substantially rectangular-shaped apertures (electron beam through holes) arranged so that the electron beams can strike the phosphors in predetermined positions.
When the electron beams impinge on the shadow mask, the shadow mask is thermally expanded. Thereby, the positions of the apertures are displaced and the electron beams passing through the apertures do not strike the phosphors in predetermined positions correctly, which can lead to color unevenness. Such a phenomenon is referred to as xe2x80x9cdoming.xe2x80x9d In order to prevent this, the shadow mask is stretched and held by a mask frame in a state in which tension is applied to the shadow mask in advance so as to absorb the thermal expansion due to the temperature increase. Thus, even if the temperature of the shadow mask is increased, it is possible to reduce the amount of relative displacement between the apertures of the shadow mask and the phosphor stripes formed on the phosphor screen.
The phosphor screen can be obtained by forming and arranging black stripes having a substantially rectangular grid shape whose longitudinal direction is the vertical direction; and then sequentially forming phosphors corresponding to each color of red, green and blue in the apertures of the black stripes. The black stripes are formed through exposing to light via the shadow mask, which has been stretched as mentioned above and installed to the face panel.
Recently, with development of computer technology, etc., an image with high definition has been demanded and the pitch of display pixels is becoming finer. Namely, the width of the aperture of the shadow mask and the arrangement pitch of black stripes are becoming finer. Accordingly, there arises a problem in that color unevenness in a display image occurs over the entire screen.
The present inventors have investigated the cause of the color unevenness and found that it is because the apertures of black stripes are formed nonuniformly over the entire screen.
FIGS. 10A and 10B are partially enlarged views schematically showing black stripes formed on the inner surface of the face panel. The vertical direction on both drawings corresponds to the vertical direction of the screen of the cathode ray tube. FIG. 10A shows an ideal pattern of the black stripes. When the black stripes are formed in a line pattern including a large number of uniform parallel lines as shown in FIG. 10A, color unevenness does not occur. However, in the cathode ray tube in which color unevenness occurs as mentioned above, as shown in FIG. 10B, lightness and darkness periodically appear in lines, and thus the black stripes are formed in a checked pattern.
As a result of further investigation, the reason why the black stripes are formed in the checked pattern as mentioned above is because the shapes of the apertures of the shadow mask used at the time of formation of the black stripes are deformed into a shape different from the designed shape.
FIGS. 11A and 11B are partially enlarged views showing apertures of a shadow mask stretched and held by a mask frame. In FIGS. 11A and 11B, the arrow T illustrates tension applied to the shadow mask.
As shown in the drawings, in the conventional shadow mask, the apertures, which are formed in a substantially rectangular shape before the shadow mask is stretched onto the mask frame, are deformed when the shadow mask is stretched and held by the mask frame with the application of tension T. The aperture is deformed into, for example, a xe2x80x9cbobbin shapexe2x80x9d in which the width (width of the aperture in the direction perpendicular to the direction in which tension T is applied) of the aperture of the substantially middle portion in the direction in which tension T is applied becomes narrow (see FIG. 11A); or on the contrary, a xe2x80x9cbarrel shapexe2x80x9d in which the width of the aperture of the substantially middle portion in the direction in which tension T is applied becomes large (see FIG. 11B). The deformation of the apertures into the bobbin shape or barrel shape may lead to formation of the black stripes having the checked pattern as shown in FIG. 10B.
Furthermore, whether the aperture of the shadow mask is deformed into the bobbin shape or the barrel shape depends on the size of the cathode ray tube (i.e., the size of the shadow mask), the magnitude of tension applied, and the like. Furthermore, even on one shadow mask, the apertures may be deformed in a different way depending on the positions on the screen (i.e. a central portion versus a peripheral portion on the screen).
The shapes of the apertures of the shadow mask, which should be of a uniform rectangular shape over the entire screen, are varied into various shapes as mentioned above, by stretching the shadow mask onto the mask frame. When the shapes of the apertures of the shadow mask are different in accordance with the positions on the screen, black stripes also are formed in different patterns in accordance with the positions on the screen, that is, in the line pattern as shown in FIG. 10A in one part and in the checked pattern as shown in FIG. 10B in another part. Therefore, shapes of the phosphors of red, green and blue formed in the apertures of the black stripes become different in one screen, which may lead to a problem, for example, an occurrence of color unevenness between the central portion and the peripheral portion on the screen.
With the foregoing in mind, it is an object of the present invention to provide a cathode ray tube having less color unevenness by forming all the apertures on the entire screen to have substantially uniform shapes as desired in a state in which the shadow mask is stretched.
In order to achieve the above-mentioned object, the present invention has the below-mentioned configurations.
According to the present invention, a cathode ray tube includes a pair of supports arranged in parallel facing each other, and a shadow mask acting as a color selection electrode including a large number of apertures. The shadow mask is stretched and held by the supports in a state in which tension is applied by the supports. In the cathode ray tube, in the shadow mask before being stretched onto the supports, the shapes of the aperture are varied so that the width of the aperture in the middle portion in the direction in which tension is to be applied gradually narrows from a central portion towards both end portions in the direction parallel to the longitudinal direction of the support.
With such a configuration, by using the shadow mask that has apertures whose shapes are varied in the direction perpendicular to the direction in which tension is to be applied before the shadow mask is stretched, it is possible to obtain the apertures having uniform shapes over the entire surface in a state in which the shadow mask is stretched. As a result, a cathode ray tube with less color unevenness can be provided.
It is preferable that in the shadow mask before being stretched onto the supports, the width of the shadow mask in the direction parallel to the longitudinal direction of the support is wide in the both end portions and narrow in the middle portion in the direction in which tension is to be applied. Furthermore, it is preferable that tension applied to the shadow mask in the central portion is larger than tension applied to the shadow mask in the both end portions in the direction parallel to the longitudinal direction of the support.
As examples of the shapes of the apertures, for example, in the shadow mask before being stretched onto the supports, the apertures formed in the central portion in the direction parallel to the longitudinal direction of the support have a barrel shape having a large width in the middle portion in the direction in which tension is to be applied, and the apertures formed in the both end portions in the direction parallel to the longitudinal direction of the support have a bobbin shape having a narrow width in the middle portion in the direction in which tension is to be applied.